Cracking Properties Research Articles

Elastic wave scattering by a fluid-saturated circular crack and effective properties of a solid with a sparse distribution of aligned cracks.

Hydraulic fractures and preexisting cracks in natural aquifers and hydrocarbon reservoirs are often saturated with fluids. Understanding the elastic wave properties in such a cracked fluid-saturated medium is of importance for many physical and engineering applications such as hydrology, petroleum engineering, oil exploration, induced seismicity, and nuclear waste disposal. In this paper, the scattering of a normally incident longitudinal (P-) wave by a fluid-saturated circular crack in an infinite elastic non-porous matrix is studied. In particular, the mechanism of hydraulic conduction (including the effects of the crack permeability and fluid inertia) inside the crack is incorporated. A semi-analytic solution for this scattering problem is derived. Based on the solution and multiple scattering theorem, an effective medium model is developed to determine the velocity dispersion and attenuation due to wave scattering in an elastic matrix with sparse distribution of aligned cracks. It is shown that the effective P-wave velocity is consistent with Gassmann's theory in the low-frequency limit. The effect of crack permeability on scattering is negligible, but the effect of fluid inertia is important. Specifically, it is found that resonance phenomena can take place inside the cracks at frequencies much lower than the scattering characteristic frequency so that rapid velocity variation can occur at relatively low frequencies. The fluid viscosity plays a damping role in weakening the resonance. The effects of crack thickness and fluid compressibility on scattering dispersion are similar to those in the case of plane-strain (two-dimensional) slit crack.

Effect of Shrinkage Reducing Admixture on Shrinkage and Crack Properties of Cement Based Materials

In order to improve the dimensional stability of cement based materials, the effects of shrinkage reducing admixture (SRA) dosage on the shrinkage and crack properties of cement based materials were investigated. The hydration process of the cement pastes was tracked and monitored by hydration calorimeter and adiabatic temperature rise apparatus respectively. The action mechanism of SRA on hydration process of the cement based materials was characterized by differential scanning calorimeter (DSC). The shrinkage and crack results show that the ability of resist cracking of concrete can be effectively improved by SRA. The results of hydration calorimeter and adiabatic temperature rise indicate that the appear time of hydration temperature peak at early age was delayed and the development of hydration heat changed gently at later period by doped SRA. The results of DSC show that the release amount of hydration heat and the production of early calcium hydroxide can be delayed by SRA, however, there has no effects of SRA on the formation of cement hydration products like calcium hydroxide at the later period.

Tensile characteristics of strain-hardening cement-based composites with different curing ages

This paper investigates tensile performances of strain-hardening cement-based composites (SHCC) with different curing ages varying from 3d to 28d. Cracking properties of SHCC specimens like the evolution of the number of cracks and the crack opening under direct tension was monitored simultaneously by using the digital images correlation (DIC) technique. Results show that the ultimate tensile strain in specimens cured for 7d is higher than that in specimens cured for 3d and 28d. The strain-hardening process involves three stages such as the initial, the intermediate, and the final stage, which are determined in terms of their cracking properties. Cracking properties of specimens cured for 3d and 7d display different rules from those of specimens cured for 28d at the intermediate strain-hardening stage, leading to different shape of stress-strain curve. Curing age influences fiber bridging stress-crack opening relation (σ(δ)) and interfacial mechanical behavior between PVA fiber and the matrix. σ(δ) gained by the DIC technique is similar to that observed in the tensile testing on notched specimens. The energy-based index of specimens cured for 7d has greater margin than that of specimens cured for 3d and 28d. The interfacial bond between fiber and matrix in specimens cured for 28d is larger than that in specimens cured for 3d and 7d.

Strengthening of RC Structures by Using Engineered Cementitious Composites: A Review

This paper presents a review of the recent work assessing the performance of building structures strengthened with engineered cementitious composite (ECC). ECC characterizes tensile strain hardening and multiple cracking properties, as well as strong interfacial bonding performance with substrate concrete, which makes it a promising retrofitting material. A lot of researches have been conducted on reinforced concrete (RC) structures, including beams, columns, beam–column joints, and fire-damaged slabs, strengthened with ECC material, and an extensive collection of valuable conclusions were obtained. These strengthening systems usually combine ECC with FRP textiles or steel bars to form a composite strengthening layer. The review demonstrates that ECC strengthening can greatly improve the performance of RC structures.

Rheological properties of asphalt binder modified with recycled asphalt materials and light-activated self-healing polymers

Ultraviolet (UV), light-activated, self-healing polymers are an emerging technology that was proposed to enhance the elastic behavior of asphalt binder, while improving its self-healing properties. The objective of this study was to evaluate the effects of self-healing polymer on the rheological properties of binder blends prepared with or without recycled asphalt materials. Binder blends were prepared with two different binders (PG 67-22 and PG 70-22M), with or without recycled asphalt materials, and 5% self-healing polymer (Oxetane-substituted Chitosan-Polyurethane). High-Pressure Gel Permeation Chromatography (HP-GPC) results showed an increase in High Molecular Weight (HMW) components in the binder with an increase in stiffness through the addition of recycled materials. A further increase was observed with the addition of self-healing polymer. Fourier Transform Infrared Spectroscopy (FTIR) confirmed High-Pressure Gel Permeation Chromatography (HP-GPC) results with an increase in the carbonyl index. Furthermore, the addition of recycled materials led to an increase in the high-temperature grade and the low-temperature grade of the binder blends, while the self-healing polymer did not have a significant effect on the PG-grade. Overall, the addition of self-healing polymer led to an increase in stiffness and an improvement in the rutting performance, while it did not have a positive effect on low-temperature cracking performance. For unmodified binder (PG 67-22), self-healing polymer incorporation improved the elastic and fatigue cracking properties of the binder. However, when it was added to a polymer-modified binder (PG 70-22M) and/or binder blends containing recycled asphalt materials, the potential of this material was low to negative on the low temperature and fatigue cracking performances.

Study on the degree distribution properties of soil crack

The Dry-hot Valley Region (DVR) is one of the typical ecologically fragile areas, but also it is a key and difficult region of vegetation rehabilitation in southwestern mountain area of China. Due to the extremely hot dry climate and strong expansion/shrinkage characteristic of soil, the cracking is a kind of extremely universal phenomenon. Soil cracking accelerates the water loss by means of evaporation, intensifying the damage of soil drought to plants, so it has become is one of the important reasons for vegetation recovery. In this paper, according to complex network theory, we analyse the degree distribution properties of two typical soil-crack network in Yuanmou Dry-hot Valley Region, which includes not only the topological characteristics of the fracture network itself, but also more important study the reflected characteristics of the soil crack under different network topologies.

Engineering Properties of Asphalt Binders from Different Sources and Their Influence on Stiffness of Asphalt Concrete Mixtures

The quality and quantity of asphalt binder are crucial for proper adhesion, cohesion, and performance of asphalt concrete (AC) mixtures. The current Superpave grading system for asphalt binders provides engineers with the high and low temperature ranges at which the asphalt binder should perform satisfactorily. The objectives of this study are to document the differences in the performance of different asphalt binders with the same nominal performance grade (PG) acquired from different refineries and to investigate the impact that binder properties may have on the stiffness of the AC mixes. To that end, five PG 64-22 and five PG 70-22 binders were studied. Each binder was graded twice, in the original state and extracted from the mix conditions. The conventional performance grading tests such as the bending beam rheometer, dynamic shear rheometer, pressure aging vessel, and rolling thin-film oven were conducted on all asphalt binders. Alternative binder parameters (e.g., parameter ΔTc, viscosity) that can potentially supplement the current PG system were measured and documented. The stiffness properties of the ten mixes as measured with the dynamic modulus tests were correlated with the measured binder properties. The asphalt binders with the same performance grades yielded different cracking, rutting, and stiffness properties, which may explain the differences in their performance when used to design AC mixes.

Experimental study on seismic performance of precast LSECC/RC composite joints with U-shaped LSECC beam shells

Precast beam-column joints are key components of precast reinforced concrete (RC) frame structures. The complex reinforcement details in the joint zone due to high seismic demand have made the prefabrication and construction process in regions with high earthquake intensity difficult to perform. As a type of high-performance fiber-reinforced cementitious composites (HPFRCCs), low-shrinkage engineered cementitious composite (LSECC) exhibits stable strain hardening and multiple cracking properties with a dry shrinkage even lower than that of normal concrete. A partial substitution of concrete with LSECC can significantly improve the seismic performance and damage tolerance of beam-column joints and help simplify the reinforcement details in the joint zone. This paper proposes a novel precast LSECC/RC composite beam-column joint using U-shaped precast LSECC beam shells. Five interior beam-column sub-assemblages were tested under cyclic loading, including one monolithic and four precast specimens. Parameters including lap splice length, transverse and longitudinal reinforcement ratios were studied. The proposed construction process was proven to be both easy and fast. The performance of the precast joints were evaluated in terms of the ultimate capacity, deformation, energy dissipation, shear distortion and strength of the joint core. The test results showed that the proposed joints could effectively simplify the joint reinforcement details while maintaining the required seismic performance. The diagonal cracks in the joint zone without transverse reinforcement were well controlled. A few stirrups in the joint zone could reduce the shear deformation and improve the energy dissipation capacity. The shear strength of the LSECC joints without any transverse reinforcement were found to be similar to the nominal shear strength of well-confined normal concrete joints suggested by ACI 318-14.

Crack Resistance Properties of HPFRC Beam-Column Joints under Cyclic Load

To investigate the crack resistance properties of high-performance fiber-reinforced concrete (HPFRC) beam-column joints, quasi-static tests of twenty-four beam-column joint specimens were performed. Test specimen variables included joint types, size effect, axial compression ratio, stirrup ratio in joint, web reinforcement, and noncorner vertical reinforcement across the joint region. The influences of these factors on the crack properties of HPFRC joints were analyzed. Test results showed that the shear strength at the crack of HPFRC exterior joints with shear failure was closer to the ultimate bearing capacity. The average ratio of the cracking shear force to the peak shear force was 0.631 for the exterior joint with shear failure, and the ratio was 0.527 for the interior joint with the same size. The size effect was observed in HPFRC joint specimens, and the average shear stress at the joint crack decreased with the increase of the joint specimen size. The increase of the axial compression ratio can improve the crack resistance properties (cracking strength and crack width) of HPFRC joints. Web reinforcement and noncorner vertical reinforcement across the joint region have no evident influence on the cracking strength of joints, but they significantly affect the distribution and width of cracks in the joint region. The formulas for calculating the cracking strength and crack width of HPFRC joints were proposed based on the test results.

Stress Corrosion Cracking Properties of Steam Generator Tubing Alloys in Crevice Environment

Stress Corrosion Cracking Properties of Steam Generator Tubing Alloys in Crevice Environment

An application of BRANN and MFL methods: Determining crack type and physical properties on M5 steel sheets

Magnetic flux leakage (MFL) method is used frequently in researches on cracks on ferromagnetic materials. Recent studies on this method focused on the improvement of low power consumption and sensor sensitivity. Generally, it was determined as to how the changing characteristics of MFL signals collected through magnetic sensor and the size of amplitude value of signal were changed based on the physical properties of crack on the ferromagnetic material (width of crack, depth of crack, etc.). As different from literature, primary purpose of our study was determining how this change occurred based on the type and physical properties of an artificial crack that were with known physical properties and that were formed with M5 directed steel layers; and the secondary purpose of our study was to develop an artificial neural network estimating the type and physical properties of a crack with unknown physical properties in the light of the data obtained. Accordingly in our study, firstly a magnetic measurement system was produced consisting of a mechanical scanning system to ensure three-dimensional movement of sensor, of a data collection module to process the data from sensor and to send the same to the computer, and of a computer software to evaluate the data from computer and to record the same. Then, artificial crack samples of different types and physical properties were prepared from M5 samples in the shape of a plate. These artificial crack samples were magnetized by placing on a core that was converted into electromagnets using 50 kHz AC signal; and the surface of material was scanned one-dimensionally with position controlled fluxgate sensors. The sensors created in fluxgate sensor based on the position were examined in terms of harmonics with DSP Lock-in Amplifier; and the amplitude values of harmonics showing the biggest change were included in the computer. Following the determination of changing graphics based on the scanning lengths of MFL signals, the mathematical curves and formulas best suiting to the characteristics of such change were determined. The changes of variables in such mathematical curve formulas were analyzed based on the type and physical properties of crack. Lastly, 4 different BRANNs (Bayesian regulated artificial neural networks) were developed estimating the type and physical properties of crack that were using the MFL signals of artificial crack samples with known physical properties and that were trained accordingly and that had unknown physical properties. First of those four was used to determine the type of crack and the other three were used to find the depth, width, lower and upper sound thickness values of crack based on the type of crack. Accuracy degrees were obtained from those BRANNs that were obtained during the training stage, respectively as R = 0.998, R = 0.959 and R = 0.964. The BRANNs trained provided results corresponding to the actual for artificial crack models with unknown crack types and physical properties.

Low cycle fatigue behavior of additively manufactured Ti-6Al-4V under non-proportional and proportional loading

Experimental tests were conducted on additive manufactured Ti-6Al-4V titanium alloy to investigate the mechanical and crack properties under multiaxial cyclic loading. Selective Laser Sintering technique (SLS) was employed to fabricate four types of cylindrical hollow specimens. The typology of each specimen is defined by the orientation of the layers and by the application of a stress-relieving heat treatment after the production process. Stress-strain cyclic curves of the materials were obtained to investigate the material cyclic plastic behavior, that resulted independent of specimen variety. Strain-controlled multiaxial low cycle fatigue tests under proportional and non-proportional loading paths were carried out on the specimens. Not heat-treated specimens exhibited a higher low cycle fatigue resistance both for proportional and non-proportional loading. Drastic initial softening was detected in the majority of the tests. Additional hardening was detected in part of non-proportional tests, which is atypical for this alloy. The mutual influence of applied load and microstructural characteristics on fatigue life are finally discussed.

Atomic layer deposited Al2O3 capping layer effect on environmentally assisted cracking in SiNx barrier films

We investigated the effect of an atomic-layer-deposited alumina (Al2O3) capping layer (2 or 10 nm thick) on the environmentally assisted cracking (EAC) properties of 250-nm-thick, plasma-enhanced-chemical-vapor-deposited silicon nitride (SiNx) barrier films on polyethylene terephthalate polymer substrates, using in situ optical microscopy tensile tests and numerical modeling. The 10-nm-thick capping layer resulted in a 5% decrease in crack onset strain, corresponding to the cracking of the Al2O3/SiNx bi-layer. Even though the Al2O3 layer itself is immune to EAC, its use as a capping layer did not significantly improve the mechanical reliability of the Al2O3/SiNx bi-layer under strain in ambient conditions, except for a minor 30%-50% increase in the driving force threshold required to induce crack growth. An effective capping layer should remain un-cracked during the cracking of the underlying SiNx, and a parametric study showed that it was not possible with alumina. A high fracture energy, low elastic modulus (e.g., organic material) layer is required such that cracking only occurs in the SiNx layer, presumably expected to protect SiNx from EAC degradation.

Cracking, Microstructure and Tribological Properties of Laser Formed and Remelted K417G Ni-Based Superalloy

The K417G Ni-based superalloy is widely used in aeroengine turbine blades for its excellent properties. However, the turbine blade root with fir tree geometry experiences early failure frequently, because of the wear problems occurring in the working process. Laser forming repairing (LFR) is a promising technique to repair these damaged blades. Unfortunately, the laser formed Ni-based superalloys with high content of (Al + Ti) have a high cracking sensitivity. In this paper, the crack characterization of the laser forming repaired (LFRed) K417G—the microstructure, microhardness, and tribological properties of the coating before and after laser remelting—is presented. The results show that the microstructure of as-deposited K417G consists of γ phase, γ′ precipitated phase, γ + γ′ eutectic, and carbide. Cracking mechanisms including solidification cracking, liquation cracking, and ductility dip cracking are proposed based on the composition of K417G and processing characteristics to explain the cracking behavior of the K417G superalloy during LFR. After laser remelting, the microstructure of the coating was refined, and the microhardness and tribological properties was improved. Laser remelting can decrease the size of the cracks in the LFRed K417G, but not the number of cracks. Therefore, laser remelting can be applied as an effective method for strengthening coatings and as an auxiliary method for controlling cracking.

A study on fracture behavior of semi-elliptical 3D crack in clay-polymer nanocomposites considering interfacial debonding

In the current research, the influence of clay platelet on crack characteristics of fully exfoliated clay/polymer matrix is scrutinized. A multi-scale 3-D finite element model comprising of four phases as clay platelet, non-perfect bond interactions, interphase region and surrounding matrix is constructed to investigate semi-elliptical matrix crack properties, as a nano-notch, subjected to fracture mode I. Subsequently, the strain energy release rate is acquired in terms of J-integral parameter considering the location and dimension, geometrical variables and modeling strategy. The results imply on the pronounced effect of clay debonding on crack behavior which leads to local stiffness reduction. Furthermore, interphase characteristics such as thickness and elastic modulus, have significant influences on the critical energy release rate. The results are consistent with published literatures and the model can be invoked as a viable tool to investigate the fracture behaviors of clay/polymer nanocomposites.

Cracking and fracture properties of potato (Solanum tuberosum L.) tubers and their relation to dry matter, starch, and mineral distribution.

Potato disorders lead to a significant reduction in the yield and quality of marketable tubers. Thumbnail cracks are physiological tuber skin disorders that can significantly affect the tuber's appearance and, hence, the overall quality. The aim of this study was to characterize fracture properties of the tuber skin. Knowledge of the physiological reasons that influence the resistance of potato tubers to mechanical impacts and thus to cracking and fracturing is limited. Tuber dry matter (DM) and starch content were found to correlate with the rheological properties of tubers, which, in turn, could affect the resistance of the tubers to cracking and fracturing. Moreover, divalent cations, such as calcium (Ca) and magnesium (Mg), and their distribution in the tuber can affect the tuber's resistance to mechanical impacts via cell-wall stabilizing properties. Tubers with higher DM, starch, and Ca concentrations exhibited the highest resistance to mechanical impacts. The reason for the increased resistance of tubers with higher DM and starch concentrations to mechanical impacts is assumed to be related to a certain cell structure of these tubers, which is why considerable force is needed to damage the cell structures. The relation between higher Ca concentrations and an improved resistance of tubers to mechanical impacts is assumed to be connected with the role of Ca in linking cell-wall polymers and thus stabilizing the cell wall. © 2018 Society of Chemical Industry.

Evaluation of High‐Performance Asphalt Binders Modified with SBS, SIS, and GTR

In this study, performance properties of polymer‐modified asphalt (PMA) binders are evaluated depending on ground tire rubber (GTR) and styrene‐isoprene‐styrene (SIS). Styrene‐butadiene‐styrene‐ (SBS‐) modified asphalt binder of PG 76‐22 is used as a base binder to manufacture the rubberized PMA binder. The rubberized PMA binders are blended using SIS modifier. The binders were artificially short‐term and long‐term aged using rolling thin‐film oven (RTFO) and pressure aging vessel (PAV) procedures. Superpave binder tests were conducted on the binders through rotational viscometer (RV), dynamic shear rheometer (DSR), and bending beam rheometer (BBR). Furthermore, multiple stress creep recovery (MSCR) test and atomic force microscopy (AFM) microstructural analysis were performed. The results of this study indicated that (1) the viscosity properties seem to be highly dependent on GTR and SIS contents, (2) the addition of SIS is observed to have a significant effect on improving the rutting performances of rubberized PMA binders, (3) the cracking properties are considered to be improved significantly through SIS modification, and (4) in general, the rubberized PMA binders with SIS showed the best performance for rutting and cracking among all the binders used in this study.

A Study on the Shrinkage Cracking Properties of Concrete by Using Blast Furnace Slag Cement and Frost-Resistant Accelerator

A Study on the Shrinkage Cracking Properties of Concrete by Using Blast Furnace Slag Cement and Frost-Resistant Accelerator

Effects of using brake pad waste powder as a filler material on the rheological and mechanical properties of a sulfur modified asphalt mixture

Thiopave sulfur product has been recognized as a kind of asphalt-binder replacement agent providing the asphalt mixture with better anti-rutting performance. However, there is often a concern about its poor resistance to the moisture damage and the low temperature. This study investigated the applicability of brake pad waste (BPW) in the sulfur modified asphalt mixture as a means to enhance its moisture, cracking, and high temperature-rutting resistance properties. The laboratory manufactured BPW powder with a size less than 0.075 mm was used as filler to substitute the limestone powder with different proportions. The effect of the BPW powder on the physical and rheological properties were assessed by the penetration test, the softening point test, the ductility test, and the dynamic shear rheometer test. In addition, the effect of the BPW on the properties of the sulfur modified asphalt mixture was evaluated using the Hamburg wheel tracking test, the freezing-thawing test, the dynamic uniaxial compression test, the semi-circular bending test, and the fatigue test. The results showed that the BPW could improve the high temperature properties of the sulfur modified asphalt mixture. In general, the anti-rutting performance of the sulfur modified asphalt mixture increased as the BPW increased. Optimum anti-moisture and anti-cracking properties of the sulfur modified asphalt mixture were also found to occur at 50% BPW powder content of the whole filler content. At an applied micro-strain of less than 300 με, the sulfur modified asphalt mixture with 50% BPW powder exhibited the longest fatigue life, with an increasing trend as the micro-strain decreased. In general, 50% BPW powder content was found to be the optimal and best substitution dosage for the limestone filler at all the laboratory performance indices that were evaluated.

A methodology framework for retrieval of concrete surface crack′s image properties based on hybrid model

Regular concrete surface crack inspection is critical since cracks are one of the earliest signs of deteriorating in concrete structures. With recent digital imaging and processing techniques, computer vision-based inspection shows huge potential in this field. A novel image processing framework for automatically retrieving multiple crack properties from concrete surface images is proposed. The framework includes (1) a crack extraction algorithm which combines edge detecting and seed growing; (2) a skeleton optimizing method which contains short branch deleting and polyline approximation algorithm; (3) a hybrid crack model which classifies crack segments into disc-shaped and bar-shaped parts based on their skeleton topological information, and (4) a triad labelling method that indicates the interrelationships between different cracks. Contrast experiments showed that the proposed segmentation method can extract more complete crack segments. Real image experiments indicated that the framework is effective and helpful for subsequent concrete structure health assessment.